The present invention relates to transmission of data on a subscriber loop in a public network such as, for example, a telephone network. More specifically, the present invention provides an improvement of standard single line digital subscriber line (SDSL) technology.
A wide variety of technologies and transmission standards have been developed for transmission of data via currently existing public network resources. A substantial portion of these resources comprise copper twisted pair transmission lines. This is especially true for the final connections to individual subscribers, i.e., subscriber loops. Without other limitations such as core network filters, such copper lines can achieve practical data rates on the order of tens of megabits per second (Mbps). Of course, substantial attenuation occurs at the higher data rates thereby limiting the length of the subscriber loop which may be serviced at such rates. For example, 24 gauge copper supports reliable transmission of data at the DS1 standard, i.e., 1.544 kbps, also commonly referred to as T1, for up to 12,000 feet. By contrast, the same 24 gauge copper will only support the STS-1 standard, i.e., 51.840 Mbps, for lines of less than 1000 feet.
The term xe2x80x9cdigital subscriber linexe2x80x9d refers to a modem or modem pair connected by one or more twisted pairs having a specific data frame format and associated transmission rate. The first digital subscriber line technology, referred to as IDSL, corresponds to what is also known as basic rate ISDN. IDSL technology transmits duplex data at 144 kbps over copper lines using a 2B1Q modulation scheme. The modems multiplex and demultiplex the data stream into two B channels (64 kbps each) and a D channel (16 kbps) as described in ANSI T1.601, the entirety of which is incorporated herein by reference for all purposes.
High data rate digital subscriber lines (HDSL) are related to the earlier IDSL using the same modulation scheme to transmit data at the T1 data rate over two twisted pairs as described in ANSI Committee T1 TR-28 and ETR 152, the entireties of which are incorporated herein by reference for all purposes. A single line digital subscriber line (SDSL) is a single pair version of HDSL, i.e., transmitting data at one-half the T1 data rate, i.e., 768 kbps, over a single twisted pair. For both HDSL and SDSL and as shown in FIG. 1, data are organized into 6 ms frames 102 comprising alternating overhead and payload sections 104 and 106. The four payload sections 106 each include twelve 97-bit payload blocks 108, 96 bits (110) of which are data and one bit (112) of which represents block overhead. This works out to the well known data rate of 768 kbps. Overhead sections 104 along with bits 112 represent an additional 16 kbps for an actual transmission rate of 784 kbps.
FIG. 2 is a simplified block diagram of a standard SDSL 202 connecting a central office or public branch exchange (represented by modem 204) and a subscriber premises (represented by modem 206). The data to be transmitted enters framing circuitry 208 of modem 204 at the raw data rate of 768 kbps. Framing circuitry 208 organizes the incoming data stream into the 6 ms frames described above with reference to FIG. 1 using a 768 kHz oscillator 210 and 16 kbps of frame overhead data generated by framing circuitry 208. The framed data are then sent to bit pump 212 where, using a 784 kHz oscillator 214, they are encoded according to the 2B1Q modulation scheme and transmitted via twisted pair line 216 to the subscriber premises as represented by modem 206.
At the subscriber premises, the modulated framed data are received at the rate of 784 kbps and demodulated by bit pump 218 which is clocked by a 784 kHz oscillator 220. The demodulated data are then received by framing circuitry 222 which strips off the 16 kbps frame overhead data and decomposes the 6 ms HDSL frames into a 768 kbps data stream. Framing circuitry 222 is clocked by a phase-locked loop (PLL) recovered clock (PLL circuitry 224) derived from the incoming data stream.
As mentioned above, standard HDSL transmission is only capable of servicing subscribers on loops of 12,000 feet or less at the T1 data rate. Unfortunately, a significant number of subscribers connected to the current network infrastructure are on loops greater than 12,000 feet. In fact, nearly one-fifth of all subscribers in the U.S. are on loops longer than 18,000 feet. Other subscriber loops have impedance mismatches caused by taps which dramatically reduce the high data rate utility of the loops. Thus, many subscribers are not able to take advantage of the increasing number of on-line services being offered with standard HDSL or SDSL data transmission. Obviously, neither will such subscribers be able to receive data transmitted according to higher rate standards such as DS2 or STS-1.
While plans to significantly decrease the average subscriber loop length in most telephone networks and replace copper with fiber optics are currently being implemented, it is likely that many subscribers will not benefit from such improvements for a number of years. Because it is desirable to provide high speed data transmission to most or all telephone network subscribers as soon as possible, efforts are currently under way to devise technologies which are capable of doing so over longer loop lengths than standard HDSL and SDSL. One technique for achieving this goal is illustrated with the block diagram of FIG. 3.
FIG. 3 is a simplified block diagram of a transmission loop 302 connecting a central office or public branch exchange (represented by modem 304) and a subscriber premises (represented by modem 306). In this example, the data to be transmitted enter bit pump 308 at a sub-standard SDSL data rate, i.e., 512 kbps. The bit pump modulates the data using a clock (oscillator 310) having an appropriately low frequency for the length of subscriber loop to be serviced. In this example, the frequency of oscillator 310 is 512 kHz. The modulated data are then transmitted via twisted pair 316 at an equivalent data rate of 512 kbps. At the subscriber end, the modulated data are received by bit pump 314 which is clocked by 512 kHz oscillator 316, i.e., the same frequency as oscillator 310. Bit pump 314 demodulates the data which results in the original 512 kbps data stream.
As is immediately obvious, this technique completely eliminates the framing circuitry to avoid being constrained by the frame format of HDSL technology. Unfortunately, this departure from the standard frame format presents its own difficulties. For example, there are a significant number of commercially available chip sets and other hardware which are designed to operate according to the HDSL standard using the HDSL frame format. However, for the lower speed technique described with reference to FIG. 3., this hardware may not be used. This forces manufacturers of such hardware and service providers to provide different technologies for different subscriber loop lengths.
Even more significantly, much of the phone network infrastructure is also designed to use the standard frame format. Thus, the solution of FIG. 3 requires modification or replacement of much of the existing system. Even if this were feasible, it would result in a system in which data are transmitted in a variety of different formats using a variety of different hardware depending upon the location of the subscriber. To the extent such a system goes against the development of transmission standards, it is clearly undesirable.
Moreover, departing from the standard frame format results in a loss of some of the advantages associated with having a standard frame. For example, 1) the ability to place all or a portion of the payload data into loopback mode to remotely test the link including bit error rate tests; 2) the ability to query the remote node customer premises device(s) or intermediate repeaters for configuration, status and the device type, version, or model number and name.
Even where the length of subscriber loops in a given service area is not an issue, there are other reasons why it is desirable for service providers to have the flexibility of offering a variety of data transmission rates without departing from a standard frame format. One reason is to more closely match the needs of individual subscribers. That is, not all subscribers require or can handle data at the maximum possible rate. It is therefore in the interest of the service provider to be more responsive to the individual requirements of specific subscribers by providing the appropriate rate for each subscriber. Such flexibility may, in turn, allow the service provider to more efficiently allocate its transmission bandwidth resources.
It is therefore desirable to provide data transmission techniques which take advantage of existing hardware and transmission standards and are capable of servicing any existing subscriber loop length. It is also desirable that such techniques give service providers the flexibility of offering subscribers a variety of data rate options without departing from those transmission standards.
According to specific embodiments of the present invention, methods and apparatus are provided which employ currently available hardware and a standard HDSL data frame format to transmit data at any of a variety of reduced data rates as may be appropriate with regard to the length and/or frequency response of a particular subscriber loop, or the specific needs of a particular subscriber. A specific embodiment of the invention employs framing circuitry at both ends of the subscriber loop based on currently available chip sets designed to operate with the standard 6 ms HDSL frame format. The chip sets are programmed to maintain all of the attributes of the standard frame except a selected attribute related to the payload blocks.
According to one embodiment, the payload block width is set lower than the standard 97 bits (768 kbps of data) to correspond to one of a plurality of data rates which are multiples of 64 kbps. The clock for the framing circuitry corresponds to the same multiple of 64 kHz and may be changed to correspond to any of the plurality of data rates. Similarly, the clock for the bit pump corresponds to the appropriate multiple of 64 kHz plus 16 kHz. In this way, the 6 ms frame length along with most other frame attributes are maintained, thereby making the transmitted data compatible with current network infrastructure hardware as well as retaining the benefits associated with having a standard frame.
According to a second embodiment, a specific number of the bits in each payload block are programmed to remain unused therefore reducing data throughput and thereby the effective data rate. Like the previously described embodiment, this embodiment has the significant advantage of retaining the standard 6 ms frame. Unlike the previously described embodiment, this embodiment only requires one set of clocks for all of the possible data rates. Given that no additional clock circuitry is required, and that each data bit in the standard HDSL frame represents 8 kbps of data, this means that transmission rates may be offered in increments of 8 kbps.
Thus, the present invention provides methods and apparatus for transmitting a data stream using framing circuitry designed to generate data frames of a first duration at a first data rate. According to the invention, data frames are generated at lower data rates than the rate for which the framing circuitry was designed, i.e., the first data rate. The framing circuitry is programmed to receive and organize the data stream into a sequence of data frames each of the first duration. The sequence of data frames corresponds to a selected one of a plurality of equivalent data rates. Each of the equivalent data rates are lower than the first data rate. According to a more specific embodiment, modulation circuitry modulates the sequence of data frames and transmits the modulated data frames at a selected one of a plurality of framed data rates. First clock circuitry provides any of a first plurality of clock signals to the framing circuitry. Each of the first plurality of clock signals corresponds to one of the plurality of equivalent data rates. Second clock circuitry provides any of a second plurality of clock signals to the modulation circuitry. Each of the second plurality of clock signals corresponds to one of the plurality of framed data rates.
According to other embodiments of the invention, methods and apparatus are provided for receiving a modulated sequence of data frames. According to a specific embodiment, the modulated sequence of data frames are transmitted at a selected one of a plurality of framed data rates. Demodulation circuitry receives and demodulates the modulated sequence of data frames thereby generating a demodulated sequence of data frames each having a first duration. Framing circuitry is employed which is designed to decompose data frames of the first duration into a data stream at a first data rate. According to the invention, the framing circuitry is programmed to receive and decompose the demodulated sequence of data frames into a data stream corresponding to a selected one of a plurality of equivalent data rates. Each of the equivalent data rates is lower than the first data rate. As described above with reference to specific embodiments for transmitting data, specific embodiments may include first clock circuitry which provides any of a first plurality of clock signals to the framing circuitry. Each of the first plurality of clock signals corresponds to one of the plurality of equivalent data rates. According to still more specific embodiments, second clock circuitry provides any of a second plurality of clock signals to the demodulation circuitry. Each of the second plurality of clock signals corresponds to one of the plurality of framed data rates.